Harshness on high mileage 390

[Thomas C Potter]

68 XL 4 speed, 230K, under light load primarily in 2nd / 3rd gear, very smooth. Moderate load I sense harshness that was not there sometime ago. At idle, no change with clutch peddle in or out. In neutral, all revs smooth. I have 3 thoughts; main bearings near failure, harm balancer old/hard, motor mounts although only 30K miles but 20 years old. I have not changed the timing. Timing chain only 50K. Any good thoughts?? Engine Restorer work well??

[Stranger in the Black Sedan]

Cranking compression? Have the valves ever been done? That is getting to the mileage where there is probably more than one factor that is going to cause the engine to not run as good as new. I just bought a truck with almost 250k miles on it and it is harsh only under load too. I am not even going to try to figure out why, the new engine is already on the stand almost ready to go. I'm sure more than one cylinder is not pulling its weight anymore due to a combination of poor valve to seat seal and/or stuck or worn piston rings

[bigmerc2003]

under load sounds like compression or ignition?? Check the points, plugs, wires, coil?? I had a truck that under load had the harshness that turned out to be a miss only under load due to a slightly burnt plug wire. Nice amount of mile though, 230K

[UncleLar]

230K miles? I think you got your money's worth out of it. Pull it apart and rebuild it and drop a 428 crank in it for around 406 cu in.

[Vortech347]

Start with the basics. Pushrod engines are easy to tell if something is wrong...

First, check all your plugs. If you're not sure what you're looking for post pictures of them and we'll help ya out. Also make sure you know which plug came from what cylinder. While you have the plugs out... Do a compression test. Its very easy.

2nd, check your wires. Run the car in a closed garage (not for long) with all the lights off and see if you're loosing spark from the wires or something else... Sometimes it'll jump through the middle.

3rd, Carb, give it a good cleaning.

DO NOT USE ANY TYPE OF "Fix engine" stuff!!!!!!!! The only thing I would recommend if you want to do that is to use some seafoam down the carb. Do not put anything into your crank case other than the correct oil. Also with the milage on your engine I'd suggest stepping up your weight a hair to 10w-40.

You're plugs will tell alot of the story. Make sure you replace them while you have them out anyway. They are cheap...

[justbob]

My guess, cheap timing chain starting to loosen up? or the cam is getting a flat spot. Sure does sounds like the Z28 timing chain i just replaced, less miles than yours and way loose.

[Vortech347]

It can very well be that too. I pulled one off a 200k 5.0 doing an engine swap 3 weeks ago and omg it was loose as traci lords. Start with the easy stuff first though...

I also forgot to mention, change your cap and rotor in the dizzy, they are cheap and easy.

[Thomas C Potter]

I did an easy thing first, retarded the timing 4 degrees. Much better. Will eventually check other items, or spend time getting the 428 completed. Thnx

TP

[Dragcity]

I have a 65 T-Bird w/ 485,000 on it. Same issue. Mine needs everything, but I would start with the electronic ignition update (Petronics), new plugs, new wires, cap, coil and such.

That is my plan come spring.... maybe I can get one more season out of it.

[ImpalaSlayer]

I have a 65 T-Bird w/ 485,000 on it. Same issue. Mine needs everything, but I would start with the electronic ignition update (Petronics), new plugs, new wires, cap, coil and such.

That is my plan come spring.... maybe I can get one more season out of it.


holyyyyyyyyyyyyyyy crap! thats alota miles!!!!!!!!!!!!!!!!!!!!!!